When my family and I travel we do all the things you just mentioned.
- We fly from NH on Southwest (two free bags each) so we bring a duffel bag full of food including: muffins, snack bars, pasta, bacon (freeze it right before you leave and is thawed by the time you get there) bagels, etc. That negates paying for breakfast at the parks and dinners some nights.
- We will by eggs and milk at the little store at the resort. We also buy the one liter water bottles and a gallon or two jugs. Drink the water bottles during the day, and then fill them up throughout the day. At night fill them halfway up with water, put them in the freezer, and in the morning fill the rest of the bottle up with water. Wrap them in washcloths and put them in a backpack. Water stays cold all day because the ice doesn't melt all the way (most of the time) and will also keep the lower back cool of the person carrying said backpack.
- Prices are about normal of what you would find at a grocery store for the milk and eggs, maybe a little more expensive, but the water is about 5 bucks a bottle for the liter, but you don't have to buy a drink the rest of the trip (unless you want soda or something else).
- Parents are DVC members so we always have a kitchen though so it's more cost effective for us to do those things.
Total savings is easily a couple hundred dollars for a week which can go a long way.